1. Field of the Invention
The present invention relates to second order nonlinear optical (NLO) polyimides having benzobisthiazole-based pendant groups and processes for preparing the same, as well as chromophore moiety-forming compounds that may be used to form the benzobisthiazole-based pendant groups of said second order NLO polyimide polymers.
2. Description of the Related Art
Electro-optics is a property whereby materials change the refractive index thereof upon the application of an electric field. This change in refractive index affects the way the materials interact with light. Information is more rapidly processed and transmitted using optical signals than electrical signals. There exists a need for finding materials that alter the transmission of optical signals or serve to couple optical devices to electrical devices, i.e., electro-optic (EO) devices. Materials suitable for use in EO devices should possess nonlinear optical (NLO) properties with large electro-optic coefficients at the desired telecommunication wavelengths. Telecommunication wavelengths are those prescribed by some standard setting body such as the International Telecommunications Union (ITU).
Electro-optic waveguide devices are essential components of the emerging field of integrated optics. Electro-optic waveguide devices can be passive waveguide devices or functional waveguide devices. Examples of passive waveguide devices include optical beam-dividers, polarizers and the like. Examples of functional waveguide devices include phase modulators, Mach-Zehnder modulators, and the like. Generally, electro-optic waveguide devices, or optical waveguides in short, consist of a transparent waveguiding core (“guiding layer”) surrounded by a layer of transparent material (“cladding layer”). The guiding layer serves the important function of interacting with and affecting the propagation of light. Materials that form the guiding layer have been traditionally inorganic materials such as lithium niobate (LiNbO3), potassium dihydrogen phosphate (KH2PO4), ammonium dihydrogen phosphate and the like. These are typically single crystal materials, and lack processing capabilities. In recent years, polymeric NLO materials have seen increased application as guiding layers. Generally, polymeric NLO materials can or may have specific advantages, such as fast response time, small dielectric constant, good linear optical properties, large nonlinear optical susceptibilities, high damage threshold, engineering capabilities, and ease of fabrication.
With the advancement in the optoelectronics industry, a great number of materials for optical transmission have been developed and studied. Further, with the finding of nonlinear optical effects, researchers and manufacturers in the art have altered their primary research topic toward the exploitation of nonlinear optical materials, which inherently have the ability to alter the frequency or color of a light beam and/or the ability to photocouple at least two light beams to result in an increase/decrease in frequency and/or amplitude.
In general, nonlinear optical (NLO) materials may be classified into two categories: inorganic materials and organic materials. Examples of inorganic nonlinear optical materials include quartz, potassium dihydrogen phosphate, lithium niobate, cadmium sulfide, cadmium telluride, etc. While inorganic nonlinear optical materials have the advantages of high transparency, high nonlinear optical coefficient and high anti-abrasion property, they bear the disadvantages of high cost, difficulty in single crystal growth, low compatibility with other optical elements and so forth, thus greatly limiting the applications and development thereof. Therefore, in recent years, researchers and manufacturers in the art have endeavored to investigate and develop high polymer materials that exhibit second-order nonlinear optic properties.
Nonlinear optical (NLO) polymers with good thermal stability have been intensively studied in the past decades. These materials have potential applications in areas such as telecommunications and optical information processing (D. M. Burland et al. (1994), Chem Rev, 94: 31-75; L. R. Dalton et al. (1995), Chem Mater, 7: 1060-1081; W. Wu et al. (1999), J Polym Sci, Part A: Polym Chem, 37: 3598-3605; M. H. Davey et al. (2000), Chem Mater, 12: 1679-1693; K. V. D. Broeck et al. (2001), Polymer, 42: 3315-3322; W. N. Leng et al. (2001), Polymer, 42: 9253-9259; L. R. Dalton (2002), Adv Polym Sci, 158: 1-86; F. Kajzar et al. (2003), Adv Polym Sci, 161: 1-85; K. Clays et al. (2003), Chem Mater, 15: 642-648; S.-H. Lee et al. (2002), J Mater Chem, 12: 2187-2188; W. Leng et al. (2001), Macromolecules, 34: 4774-4779; M. Hasegawaa et al. (2001), Prog Polym Sci, 26: 259-335).
NLO polymers have several advantages over single crystalline inorganic and organic molecular systems. These include ease of preparation, adjustable refractive indices and controlled arrangement of spatial order. For second order applications, it is imperative that the material be noncentrosymmetric. In noncentrosymmetric organizations, several organic molecular and polymeric systems have been characterized by large second order NLO coefficients, ultra-fast response times, performance over a broad wavelength range and high laser damage threshold, as compared to the traditional inorganic materials, e.g., lithium niobate or potassium dihydrogen phosphate.
NLO polymers can be cast as films on substrates by processes such as spin coating from a solution of the polymer in a solvent, spraying, Langmuir-Blodgett deposition, and the like. The substrate materials employed for electro-optic waveguide devices are generally inorganic materials such as silicon, GaAs, GaAlAs and the like. Silicon is particularly preferred as substrate material due to its ready availability in wafer form in a well-purified state, and the highly-developed state of its technology in integrated circuit and electronics industries. Wafers from silicon also have the advantage that they can be easily cleaved into minute chips carrying the individual devices.
Many of the current organic polymeric NLO materials are prepared by blending a NLO molecule into a polymeric host material. The term “blending” as used herein means a combination or mixture of materials without significant reaction between specific components. However, a problem associated with “guest-host” polymers with NLO properties produced by a simple blending of NLO molecules into a host polymer is that these polymer materials lack stability of orientation. Generally, the incorporation of molecular structures having NLO activity into the backbone of a polymer chain will decrease the likelihood of the structural reorganization in comparison with polymers in which the NLO active molecule is simply blended. It is therefore desirable to provide a polymer material with NLO groups covalently bonded to the backbone thereof to minimize relaxation effects.
A number of approaches have been made in the past decades to organize NLO molecules in a polymer matrix in a noncentrosymmetric manner. The most important, but not the only aspect from the standpoint of application, is the organization of NLO molecules into preferred orientation and their stability in the aligned state up to at least cold wire bond temperatures (about 100° C.). Historically, one of the first approaches to this alignment of NLO molecules in a polymeric system came with the concept of the guest-host system. The NLO molecules may be incorporated by a solution casting method with an amorphous polymer, and the second order non-linearity may be imparted by subsequent poling of the NLO molecules in the polymer matrix using an external electric field, e.g., corona poling, parallel plate poling or integrated electrode poling. Advantages of this approach are ease of processing, tailorable refractive indices, control of spatial ordering of the polymer, and choice of a wide range of materials. However, the decay (both the initial and long term) of second order properties as confirmed through second harmonic generation (SHG) from the polymer matrix is unavoidable when the poling field is withdrawn from the polymer matrix. Moreover, a high degree of loading of the NLO molecules into the host polymer is not possible because of phase segregation of the polymer matrix or blooming of NLO molecules in the matrix, both resulting in optical scattering.
In a second approach, known as “grafted” systems, a number of new features are routed just by linking NLO molecules covalently in the side chains of a suitable polymer backbone. Despite the synthetic complexity of such a system, a large number of NLO molecules (a concentration 2 to 3 times greater than that in the guest-host system) can be coupled with the polymer side chains, and the polymers are still easily processable. Both the initial and long term decay in second harmonic (SH) properties are reduced to a great extent.
A three dimensional network consisting of NLO molecules, known as the “cross-linked” system, has been developed to overcome a number of problems associated with the guest-host or grafted systems. In this system, multifunctional epoxy and amino compounds containing NLO components are simultaneously processed, poled and cross-linked to freeze-in the nonlinear effects permanently. Properties resulting from the cross-linked system are significantly small decay in SH properties over a long period of time and the ability for processing with large concentrations of NLO molecules. However, to develop an optimal epoxy-based NLO material, precise control of the molecular weight of the prepolymer is a stringent and necessary condition. In addition, poling and curing at elevated temperatures have to be carried out over a long period of time (about 20 hours), making processing of the materials significantly difficult.
There are other nonlinear optical polymers, which contain nonlinear optical moieties that form parts of the polymer backbone or are appended to the polymer backbone through intervening spacer groups. Nonlinearity of moieties is described in terms of second order nonlinearity, third order nonlinearity, and so on, with the corresponding unit values being referred to as second order nonlinear optical susceptibility, third order nonlinear optical susceptibility, and so on. Nonlinear optical moieties of polymers that are preferred to act as guiding layers in optical waveguide devices generally must possess acceptable second order nonlinear activity. These moieties are generally made up of conjugated π-electron systems with an electron donating group such as an amine group, and an electron-acceptor group such as a nitro group forming either end of the conjugated π-electron system.
Second order nonlinear optical (NLO) polymers are expected to find extensive uses in opto-electronic applications. For example, a number of applications, such as second harmonic generation (SHG), frequency mixing, electro-optic modulation, optical parametric emission, amplification and oscillation have been proposed for organic and polymeric materials with large second order NLO coefficients.
Although second order nonlinear optical (NLO) polymers hold promise for applications in electro-optical devices, a number of issues have to be addressed before they can see wider commercial application. Three of these crucial issues are the high temporal stability of dipole orientation, large optical nonlinearity and minimum optical loss. Due to a realization of the intrinsic nature of the optical loss (due to C—H overtone vibration absorption), major research efforts have been focused on optimizing the optical nonlinearity and stabilizing the dipole orientation.
Different approaches have been undertaken to address these issues, and considerable progress has been achieved. For example, various cross-linking schemes (photochemical and thermal cross-linking) have been developed to lock the dipole orientation in the polymer matrix after electric poling. Temporal stabilities of second order NLO activity thus have been enhanced. The rationale behind the design of these polymers is that after cross-linking, the motion of the free volume in the polymer matrix can be frozen. This is reflected in the increase in glass transition temperatures of the resultant NLO polymers. The same notion leads to the concept that as long as a polymer has a high glass transition temperature, the induced dipole orientation can be stabilized within a certain temperature range. This was clearly demonstrated in second order NLO polyimide composite materials.
For example, in U.S. Pat. No. 5,399,664 issued to Zhonghua Peng and Luping Yu, polymers that exhibit second order nonlinear optical properties and characterized by high-temperature stability are disclosed to be prepared by a polycondensation reaction between an aromatic dianhydride and a compound selected from the group consisting of a di(alkylamino)amine and an aromatic diamine. According to this US patent, a most preferred polymer is a product of a polycondensation reaction between 1,2,4,5-benzenetetracarboxylic dianhydride and nitro(N,N-diethylamino)stilbene. Said polymer can then be heated under conditions sufficient to form a polyimide polymer.
U.S. Pat. Nos. 5,433,895 and 5,676,883 issued to Ru J. Jeng et al. disclose a nonlinear optical composition which includes a silicon-containing component and a nonlinear optical component, wherein the nonlinear optical component includes an alkoxy-silane azo dye. In these two US patents, there is also disclosed a method for forming a nonlinear optical composition, comprising the steps of: (a) forming a sol of an alkoxy-silane compound that includes a nonlinear optical component; (b) exposing the sol to conditions sufficient to form a gel; (c) poling the nonlinear optical component, whereby a nonlinear optical composition is formed which exhibits nonlinear optical activity; and (d) exposing the gel to conditions sufficient to cause transesterification of the alkoxy-silane compound, thereby forming the nonlinear optical composition.
In connection with polymers having NLO properties and their uses in fabricating optical elements, available references include, but are not limited to, U.S. Pat. No. 5,532,320, U.S. Pat. No. 5,594,093, U.S. Pat. No. 5,688,906, U.S. Pat. No. 5,814,833, U.S. Pat. No. 5,834,575, U.S. Pat. No. 5,837,804, U.S. Pat. No. 6,294,593 B1, U.S. Pat. No. 6,340,506 B1, U.S. Pat. No. 6,503,998 B2, U.S. Pat. No. 6,894,169 B1, etc.
There is a continuing effort to develop new nonlinear optical polymers with increased nonlinear optical susceptibilities and enhanced stability of nonlinear optical effects, in particular second-order NLO polymeric materials.
Several factors are required for the practical application of second-order NLO polymeric materials, including large molecular hyperpolarizability (β), and satisfactory optical transparency, photochemical and thermal stability. The NLO polymeric materials must exhibit high thermal and oxidative stability, low optical loss, good processability and low chain segment mobility.
Amongst NLO polymers with side-chain chromophores, polyimide is a good candidate due to its high chain stiffness, and good thermal and high oxidative stability, making it possible to maintain the orientational stability of side-chain chromophores (D. M. Burland et al (1994), Chem Rev, 94: 31-75; W. Wu et al. (1999), J Polym Sci, Part A: Polym Chem, 37: 3598-3605; M. H. Davey et al. (2000), Chem Mater, 12: 1679-1693; K. V. D. Broeck et al. (2001), Polymer, 42: 3315-3322; W. N. Leng et al (2001), Polymer, 42: 9253-9259; T. D. Kim et al (2000), Polymer, 41: 5237-5245; Y. Sakai et al (1999), J Polym Sci, Part A: Polym Chem, 37: 1321-1329; H. Q. Xie et al (1998), Polymer, 39: 2393-2398; D. Yu et al (1995), J Am Chem Soc, 117: 11680-11686). These chromophores can be conveniently incorporated into the polymer backbone after being poled at elevated temperatures. Several NLO polyimides based on 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6 FDA) and some chromophore-containing diamines have been reported in various literature (see, e.g., M. H. Davey et al. (2000), Chem Mater, 12: 1679-1693; K. V. D. Broeck et al. (2001), Polymer, 42: 3315-3322; W. N. Leng et al. (2001), Polymer, 42: 9253-9259; H. Q. Xie et al. (1998), Polymer, 39: 2393-2398; D. Yu et al (1995), J Am Chem Soc, 117: 11680-11686). It is also well known in the art that the presence of the CF3 group in the backbone chains of polyimide polymers can greatly improve the solubility of said polymers in organic solvents. Such fluorinated NLO polyimide polymers also exhibit good processability for film casting.
The molecular design of a NLO chromophore should contain an electron donor, a π-electron connective bridge, and an electron acceptor (D. M. Burland et al. (1994), Chem Rev, 94: 31-75; L. R. Dalton et al. (1995), Chem Mater, 7: 1060-1081). The optical nonlinearity of a chromophore can be evaluated by μβ value from simple calculation with two-level mode, where μ is the difference between the dipole moment between the ground and excited state (D. M. Burland et al. (1994), supra). During the 1980s, most researchers focused on finding stronger donor and acceptor groups and on increasing the length of the π-electron connective bridge (L. R. Dalton et al. (1995), supra).
However, in 1991, S. R. Marder and his co-workers demonstrated that an increase in the donor-acceptor strength would lead to a diminution of hyperpolarizability (S. R. Marder et al. (1991), Science, 252: 103-106). They then focused on two design factors in optimizing hyperpolarizability: firstly, a chromophore should have a π-electron connective bridge that loses aromaticity upon polarization, and an acceptor that gains aromaticity upon polarization, so that the bond length alternation of the π-electron connective bridge could be reduced, resulting in a significant enhancement in μρ values; and secondly, the optical nonlinearity could be improved by replacing benzene rings with heterocyclic rings such as thiazole or thiophene, the structures of which have less aromatic stabilization energy, so that chromophores containing the same in the π-electron connective bridge(s) could have larger μβ values than those containing two benzene rings.
Considerable efforts to incorporate benzothiazole-derived chromophores into polyimides as side chains have been made. These benzothiazole-based polyimides exhibited attractive NLO activities and good thermal stability (W. N. Leng et al. (2001), Polymer, 42: 9253-9259; Y. Sakai et al. (1999), J Polym Sci, Part A: Polym Chem, 37: 1321-1329; H. Q. Xie et al. (1998), Polymer, 39: 2393-2398).
For example, in W. N. Leng et al. (2001), Polymer, 42: 9253-9259, there is disclosed a NLO polyimide polymer with side-chain chromophores, which was prepared from the following reaction:

Said NLO polyimide polymer was found to have an electro-optic coefficient (r33) of 22 pm/V at 830 nm and a glass transition temperature (Tg) of 248° C. However, due to the presence of azo group in the pendant chromophore, the NLO polyimide polymer has poor thermal stability and the second order nonlinear optical property thereof will be drastically reduced and may even be lost at high temperatures.
Polymers other than polyimides have also been considered. For example, it was reported that poly(p-phenylenebenzobisthiazole) (PBZT) has promising nonlinear optical properties (J. A. Osaheni et al. (1992), Chem Mater, 4: 1282-1290). In addition, it was reported that poly(methyl methacrylate) (PMMA) containing 2 mol % of 6-(perfluoroalkyl)benzothiazolylazo dye has a larger second-order nonlinear optical coefficient (d33) than PMMA with the same loading amount of Disperse Red 1 (DR 1) dye (M. Matsui et al. (1998), Dyes and Pigments, 38 (1): 57-64).
On the other hand, in connection with the studies of chromophores, S.-H. Lee and his co-workers synthesized a series of planar rigid-rod push-pull 2,6-diphenylbenzo[1,2-d:4,5-d]bisthiazole (DPBBT) derivatives, using 2,5-diamino-1,4-benzenedithiol dihydrochloride as the starting material:
wherein A represents CN or NO2, and D represents N(CH3)2 or N(CH3CH2)2.
The above derivatives were reported to be thermally stable, to have highly efficient second-order nonlinear optical nonlinearities, and to exhibit optical transparency down to 532 nm (S.-H. Lee et al. (2002), J Mater Chem, 12: 2187-2188). However, these derivatives are not susceptible of industrial applications due to the poor solubility thereof. Therefore, some investigators in the art have endeavored to modify the molecular structures of such chromophores or utilize the same in the studies of NLO polymeric materials.
In earlier studies of the applicants, six poly(benzobisimidazoles) (PBIs) and six poly(benzobisthiazoles) (PBTs) were synthesized by the solution polycondensation of 1,2,4,5-tetraaminobenzene tetrahydrochloride (C. C. Chen et al. (2002), J. Mater. Sci., 37 (19): 4109-4115) and 2,5-diamino-1,4-benzenedithiol (http://polymer.che.ncku.edu.tw/papers/B-Functional/B104.pdf) with systematically varied diacids in poly(phosphoric acid) (PPA), respectively. Based on the findings from these studies, the applicants endeavored to design new second order NLO polymers that are easy to prepare, that have a glass transition temperature (Tg) greater than 200° C., and that exhibit satisfactory NLO properties.